1. Field of the Invention
The invention concerns a process for the manufacture of aqueous polyisocyanate emulsions. More particularly, the invention concerns an improvement in the process for the manufacture of aqueous polyisocyanate emulsions wherein water-soluble polyamides are employed as emulsification agents.
2. Description of the Prior Art
The use of isocyanates as chipboard adhesive is well known. Particularly well known are binders based on diphenylmethane-4,4'-diisocyanate. As solvent-free liquid products, they can be used in the processing facilities employed in the wood industry.
One drawback, particularly in the case of low concentrations, is that it is difficult to distribute the isocyanate-based adhesives uniformly on the chip material. This can cause a variation in the properties of the finished chipboard. Another drawback is that vessels and/or machines contaminated with isocyanate cannot be cleaned simply with water. It is known that organic solvents can be used to dilute isocyanates. However, suitable solvents such as dichloromethane and others cannot be used for toxicological reasons and other solvents such as acetals are not practical because of the fire hazard.
In German Published Application No. 2,610,552, the use of aqueous emulsions of isocyanates as adhesives for lignocellulose-containing particles was suggested.
However, special requirements must be met by the isocyanates themselves and the emulsifying auxiliaries as indicated by the result of the experiments treated in the German Published Application. Prepolymers, that is, reaction products of polyisocyanates with polyester and/or polyether polyols, which still have reactive isocyanate end groups are preferably used as (poly)-isocyanates. Nonionic products free of reactive hydroxyl-, amino- or carboxylic ester groups are used as emulsifiers and/or surface active agents.
As the German Published Application admits, however, even with the use of considerable quantities of special hydrophobic polyols which are used for the manufacture of the above-referenced prepolymers, it is not possible to obtain emulsions which have a sufficient service life. Example 2 of the German Published Application indicates that the loss of isocyanate groups in the emulsion during two hours at room temperature is approximately 14 to 17 percent in the most favorable case and more than 50 percent in the less favorable case.
For the expert engaged in the manufacture of wooden products, this means that such an adhesive is without practical value since the required service life between mixing an emulsion and use, which is based on the capacity of the storage vessels for adhesive and adhesive-coated chip goods, cannot be achieved.
German Patent Application No. P 2703271 is directed to a process of manufacturing aqueous, storage-stable polyisocyanates wherein the polyisocyanates are dispersed and/or emulsified in water in the presence of an emulsifier based on a polyglycol having a molecular weight of approximately 4,000 to 20,000. There is no teaching in the prior art of the use of water-soluble polyamides as emulsifiers for aqueous polyisocyanate solutions.
The purpose of this invention, therefore, lies in creating isocyanate preparations which either do not have the referenced drawbacks or display them to a considerably lesser degree. More specifically, the purpose of this invention is the creation of improved storage-stable aqueous emulsions of polyisocyanates.